1. Field of the Invention
The present invention relates to a turbo-molecular pump.
2. Description of the Related Art
Heretofore, a turbo-molecular pump has been used as a pump for evacuating an inside of a process chamber to perform a given treatment in a high vacuum atmosphere during a semiconductor manufacturing process. The turbo-molecular pump comprises a rotor which has a plurality of rotating blades formed in an outer peripheral surface of a bell-shaped cylindrical body thereof, in a multistage arrangement. In view of a need for each of the rotating blades to achieve a higher compression ratio as it is located on a more downstream side, a downstream cylindrical body provided with a part of the rotating blades located in a downstream region (i.e., downstream rotating blades) is designed to have an outer diameter greater than that of an upstream cylindrical body provided with the remaining rotating blades located in an upstream region (i.e., upstream rotating blades), in order to provide a higher peripheral speed to a root portion of each of the downstream rotating blades (see, for example, JP 2006-090231A).
In a portion of the upstream cylindrical body integrally connected to the downstream cylindrical body, the downstream cylindrical body is applied thereto as an additional mass to cause an increase in stress in a downstream end of the upstream cylindrical body. Moreover, there is a problem that, when it is attempted to increase a rotation speed of the rotor so as to obtain enhanced evacuation performance, an upper limit of the rotor rotation speed will be undesirably restricted by the stress in the downstream end.